Hi,
Please see this example below :

This is very simple example, why we need Read List. Explanation :
1. This workflow is scheduled every 15 min. The requirement is that it should only read the data from the database only once when it starts first time. Then it should send the delivery every 15 min to same group of recipients which came in first run of workflow.
So, how to achieve this, because data in the initial query (1. Query) may change everytime you run the workflow.
2. So, to implement this, when we will start the workflow first time, we will create a list of that data in the List Update Activity (2. List Update).
3. Then we will simply read the list again and again after sending the delivery each time by implementing some logic. So, that only the same group of recipients will receive the delivery.
Note # This workflow is just to clear the idea behind Read List to you, it may or may not run.
I hope this clears your doubt.
Regards,
Kapil Sharma